1. Field
One embodiment of the invention relates to a magnetic head configured to record information to a recording medium, such as a magnetic disk, and an information storage device mounted with the magnetic head.
2. Description of the Related Art
A lot of information is routinely processed in the field of computers. A hard disk drive (HDD) is used as one type of information storage device for recording and reproducing such information. The HDD contains a magnetic disk, for use as a disk storage medium in which information is recorded, and a magnetic head for recording information to the disk.
In recent years, the recording density of HDDs has been increasing with the reduction of the distance between a magnetic head and magnetic disk (i.e., flying height of the head) during information recording or reproduction. Currently, the flying height of the magnetic head is set to about 10 nm. It is generally known, however, that the flying height is not constant.
For example, the head flying height varies depending on changes in temperature, atmospheric pressure, etc., around the magnetic head. The head generally comprises a coil through which a current corresponding to information to be recorded is passed during recording. If the current is passed through the coil to perform the recording, the coil produces heat and thermally expands. When this is done, the degree of expansion of the coil changes as the current corresponding to the information to be recorded changes. Accordingly, the flying height of the head changes with the change in the degree of thermal expansion.
Such a change in the flying height may cause a collision between the magnetic disk and head, reduction in recording performance, etc. Thereupon, techniques for thermal deformation are proposed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 5-20635, U.S. Pat. No. 5,991,113, and Jpn. Pat. Appln. KOKAI Publications Nos. 2004-335069, 2008-27504, and 2005-285236. According to these techniques, a heater is built in a magnetic head, and its heating value is suitably adjusted so that the head can be thermally deformed to compensate for the change in its flying height.
In this case, the magnetic head is mounted on a slider, which is formed with an air-bearing surface whereby the head is caused to fly to the aforesaid flying height above the magnetic disk. Since the air-bearing surface of the slider is subject to manufacturing errors, the flying height of the head frequently varies according to each manufactured head. The variation of the flying height can be eliminated by adjusting the flying height by the aforementioned heating.
According to the above-described techniques in which the flying height of the magnetic head is adjusted by the thermal deformation by means of the heater, if electric power supplied to the heater is not higher than a certain level, the thermal deformation that is caused depending on its level is canceled when the power supply to the heater is stopped. If the power supplied to the heater is high, however, a stress heavier than the yield point may act on a part of the magnetic head, thereby causing plastic deformation. If such plastic deformation occurs, a protrusion (residual pole tip protrusion or PTP) inevitably remains on a part of the head after the power supply to the heater is stopped. This residual PTP serves to reduce an unadjusted flying height before the power supply. If the residual PTP is large, therefore, an allowance (heater stroke) cannot be secured for the adjustment based on the thermal deformation by the heater, so that the adjustment inevitably becomes impossible. The higher the power supplied to the heater, the larger the residual PTP is. In many cases, according to the aforementioned techniques, therefore, an upper limit is provided for the power supplied to the heater, in order to suppress the residual PTP. Actually, the residual PTP is large compared to the power supplied to the heater, so that the upper limit of the supplied power cannot be set to be very high.
As mentioned before, the flying height, which depends on the air-bearing surface of the slider on which the magnetic head is mounted, frequently varies according to each head because of manufacturing errors. According to the aforementioned techniques, this variation in the flying height attributable to the manufacturing errors can be eliminated. If the original flying height before the thermal deformation is great, however, the variation may not be able to be eliminated by supplied power lower than the upper limit. In many cases, those magnetic heads whose flying height variations are too great to be eliminated by supplied power lower than the upper limit are discarded as defective products, so that the yield rate of heads is reduced. Accordingly, there is a demand for the development of magnetic heads of which the residual PTP relative to the power supplied to the heater is suppressed.